Codex4

For serious language learners

Get fluent faster by learning the words you actually use.
Codex4 helps you build a usable vocabulary and keep it for the long term.
- Learn what you need: Add your own words from any text in one tap. Build vocabulary for your job, studies, or hobbies.
- Retain more with less effort: Reviews are timed by FSRS (a modern spaced-repetition model) so you study exactly when it counts.
- Master real usage: The app treats each word as a lemma, so forms and conjugations are covered naturally as you progress.
- Stay organized and motivated: Clear stats show what’s due, what’s known, and what’s next.
- Bring any content: Automatic word extraction and a context-aware lemmatizer handle homonyms and multi-word phrases.
- Your data stays yours: Keep access to review even after unsubscribing, and export your words and learning data anytime.
- No gimmicks: Fast sessions, clean design, built for serious learners.